Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Image-wise or information-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
3. Direct thermal formation of a visible image pattern by image-wise or information-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
Thermographic materials of type 3 become photothermographic if a photosensitive agent is present which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Examples of photothermographic materials are the so called "Dry Silver" photothermographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
DOS 2 248 545 discloses a copying process characterized in that the whole surface of a thermally developable photosensitive silver material is subjected before the complete execution of the exposure to a uniform auxiliary thermal treatment at a temperature, that is within the suitable development temperature range for the photosensitive material, and to thermal development after exposure; and that, in a subclaim, the photosensitive material can also be subjected to auxiliary heating during the exposure thereof. The specification and invention examples give no indication of the efficacity of auxiliary heating during exposure.
Yu. E. Usanov, T. B. Kolesova, L. P. Burleva, M. R. V. Sahyun and D. R. Whitcomb in the preprints of the Society for Imaging Science and Technology (I.S.& T.) 50th Annual Conference held at Cambridge, Mass., USA between the 18th and 23rd May 1997, pages 42 to 45 reported that: Experiments carried out with thermally developed Dry Silver.TM. type 7858 photothermographic materials from 3M and spectrally sensitized model photothermo-graphic materials showed that preheating at temperatures higher than 100.degree. C. followed by cooling prior to flash lamp or sensitometer exposure results in decreased speed during development. Consequently, a decrease in the photosensitivity was observed. Analogous results were also obtained if the materials were exposed during the preheating process. The higher the pre-heating temperature above 100.degree. C. and the longer the time of preheating, the lower the material photosensitivity. It was determined that this phenomenon depends neither on the composition of the silver halides nor on the method of incorporating them into the thermally developable composition.
It is desirable to increase the photosensitivity of photosensitive thermally developable photographic materials to be able to use less powerful heat sources. It is also desirable to achieve such an increase in photosensitivity, while enabling the simplification of photothermographic processing equipment, for example as disclosed in DE-A 196 36 253.0.